NANO IDEA Open Access

High-Efficient Liquid Exfoliation of BoronNitride Nanosheets Using Aqueous Solutionof AlkanolamineBangwen Zhang1,2*, Qian Wu1, Huitao Yu1, Chaoke Bulin1, He Sun1, Ruihong Li1, Xin Ge1 and Ruiguang Xing1

Abstract

As one of the simple and efficient routes to access two-dimensional materials, liquid exfoliation has receivedconsiderable interest in recent years. Here, we reported on high-efficient liquid exfoliation of hexagonal boronnitride nanosheets (BNNSs) using monoethanolamine (MEA) aqueous solution. The resulting BNNSs were evaluatedin terms of the yield and structure characterizations. The results show that the MEA solution can exfoliate BNNSsmore efficiently than the currently known solvents and a high yield up to 42% is obtained by ultrasonic exfoliationin MEA-30 wt% H2O solution. Finally, the BNNS-filled epoxy resin with enhanced performance was demonstrated.

Keywords: BN nanosheets, Monoethanolamine, Liquid exfoliation, Yield, Composite

BackgroundSince the discovery of graphene in 2004, the interests ingraphene and its analogues two-dimensional materials[1, 2] are ever growing throughout the world. Single-layer or few-layer boron nitride nanosheets (BNNSs),known as “white graphene,” share near identical struc-ture to graphene that the sp2 hybridized B and N atomscovalently bind into hexagonal crystal in single layers,resulting in weak van der Waals forces between them.Due to the structure and a wide band gap (5.5 eV) [2],BNNSs are endowed with outstanding mechanical,thermal, and dielectric properties, as well as excellentchemical stability, thus exhibiting great potentials inapplications such as transparent films [3, 4], protectivecoating [5, 6], advanced composites [7–9], dielectrics[10, 11], and electronic devices [12, 13] etc.To produce ultra-thin BNNSs, a variety of methods

such as ball milling [14–16], intercalation-oxidation [17,18], chemical vapor deposition (CVD) [3, 4], and liquidexfoliation [2, 19–28] have been developed. Of thesemethods, both ball milling and intercalation-oxidationmethods are time-consuming and prone to induce

impurity and defects in samples, while the CVD costshighly and is applied to prepare continuous film insteadof disperse nanosheets that are more popular in practicalapplications. Recently, liquid exfoliation of BNNSs fromhexagonal boron nitride (hBN) powder has receivedmuch attention because it is easy to use, economical,free of defect, etc. The driving forces were ascribeddynamically to sonic vibration [19, 20] or liquid shear[21, 22], and thermodynamically to minimization ofGibbs mixing free energy [23, 24] or interfacial energy[25] between the nanosheets and solvents. According tothe latter, the composition and properties of usedsolvent play an important role in liquid exfoliation.Much research has shown that hBN can be exfoliatedpreferentially in few pure solvents such as N-methyl-2-pyrrolidone (NMP), dimethylformanmide (DMF), and iso-propanol (IPA) [9, 19–24, 26] and some mixed solvents[25, 27–29]. However, high-efficient and cheap solventsfor hBN liquid exfoliation have been rarely reported, limit-ing the large-scale preparation and applications of BNNSs.In the present paper, monoethanolamine (MEA) aque-

ous solution was attempted for the first time for liquidexfoliation of BNNSs. It was found to exfoliate hBNmore efficiently than the other solvents with very highyield. Moreover, this solution has higher specific surfacetension (SST) than that of known solvents. The obtainedBNNSs were characterized by X-ray diffraction (XRD),

* Correspondence: bangwenz@126.com1School of Materials and Metallurgy, Inner Mongolia University of Scienceand Technology, Baotou 014010, China2Instrumental Analysis Center, Inner Mongolia University of Science andTechnology, Baotou 014010, China

© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link tothe Creative Commons license, and indicate if changes were made.

Zhang et al. Nanoscale Research Letters (2017) 12:596 DOI 10.1186/s11671-017-2366-4

scanning electron microscopy (SEM), transmissionelectron microscopy (TEM), Raman, and X-ray photo-electron spectroscopy (XPS) techniques. Finally, as anexample, the BNNSs were employed to reinforce epoxyresin (ER). The obtained composites exhibited improvedthermal and mechanical properties.

Results and DiscussionFigure 1a compares six solvents with respect to the yieldand suspension concentration of exfoliated BNNSs. It isobvious that among these solvents, MEA earns the high-est yield up to 33.7%. The resulting suspension is milk-white (see the inset) with a high concentration of1.3 mg/mL. In contrast, the yield of other solvents is12% (DMF), 9.5% (NMP), 8.4% (tBA), 4.5% (IPA), and1.5% (H2O), far below that of the former. The suspen-sions produced from these solvents are transparent orsemi-transparent because of noticeable precipitation. Toevaluate the stability of exfoliated dispersion, we mea-sured the UV-vis absorbance (400 nm) normalized to itsinitial value (A/A0) of the MEA-exfoliated suspensiondependent on storage time, as shown in Fig. 1b. Forcomparison, the absorbance of NMP-exfoliated suspen-sion was given together. It shows both two suspensionsare stable, so the A/A0 retains about 90 and 86% respect-ively after standing for 50 h, when Tyndall scattering isstill clear, as shown in the inset of Fig. 1b.To further investigate the exfoliation in MEA aqueous

solution, the yield and suspension concentration of exfo-liated BNNSs dependent on mass percent of water withthe corresponding specific surface tension (SST) wereplotted in Fig. 2a. With the increase of water content,the yield first decreases and then increases to a peakvalue, followed by a drop in succession till near zero inpure water. The highest yield of 42% that corresponds tothe suspension concentration of 1.5 mg/mL wasachieved in MEA-30 wt% H2O solution with a high SSTmore than 50 mJ/m2. To our best knowledge, this yieldis possibly the highest value reported in literatures for

liquid exfoliation of BNNSs and other two-dimensionalmaterials (Additional file 1: Table S1). Even compared toother exfoliation methods (Additional file 1: Table S2),such as intercalation and ball milling exfoliation, thisresult is still highly competitive. Also, the MEA solutioncan keep a higher exfoliation yield (more than 30%)when its water content varies widely from 20 to 60 wt%,implying that BNNSs can be prepared more economic-ally in this solution than other pure solvents. Similarly,we compared the stability of two suspensions exfoliatedby MEA-30 wt% H2O and NMP-30 wt% H2O respect-ively in Fig. 2b. As compared to Fig. 1b, they showhigher concentration and increased absorbance due tothe increased yield, the corresponding absorbanceincreases by 8 to 98% and by 5 to 91%. This indicatesthe stability of exfoliated product increases due to theaddition of water. From now on, we specify the BNNSsas those exfoliated by MEA-30 wt% H2O.Now two problems arise from above observations:

first, how to understand the superior exfoliationperformance of MEA compared with other solvents;second, why can the introduction of water in appropriateamount in MEA improve the exfoliation? Regarding thefirst problem, we resort to the solubility parametertheories (SPTs). Following these theories, Coleman et al.[23, 24] suggested that the solvents for effective exfoli-ation were those with dispersive, polar, and H-bondingsolubility parameters matching those of layered materialsin order to minimize the exfoliation energy. They foundthat hBN was most effectively dispersed in thosesolvents with a SST close to 40 mJ/m2. In other studies[16, 28], this value was reported as 20~40 mJ/m2. In oursystem, pure MEA has a SST of 44.8 mJ/m2 accordingto ref. [30] (where the data at 50 °C was taken accordingto the experimental condition, the same below), whichroughly agrees with this case. However, when MEA ismixed with 20~60 wt% of water (the second problem),an enhanced exfoliation was observed in this solution,whose SST is about 49~55 mJ/m2 (Fig. 2a) and much

Fig. 1 Comparison of a the yield and suspension concentration of BNNSs exfoliated in various solvents and b normalized absorbance of theMEA- and NMP-exfoliated suspensions vs. storage time

Zhang et al. Nanoscale Research Letters (2017) 12:596 Page 2 of 7

higher than the previous values. This enhanced exfoli-ation which occurred in high SST of mixed solvents ispresumably due to following factors: (1) MEA moleculesthat tend to form a network or ring-like structure due tothe interactions among amino and hydroxyl groups aredisaggregated by the added water molecules [31], allow-ing them to intercalate BN layers more easily andenhance the exfoliation; (2) the water makes the aminogroups of MEA absorbed on BNNSs hydrolyzed andincreases surface potential of the BNNSs, hence introdu-cing an additional electrostatic stability, as observed inMEA-30 wt% H2O-exfoliated BNNS suspension(Fig. 2b); (3) more or less addition of the water woulddeviate from the condition above and restrict theliquid exfoliation.Figure 3a, b displays the SEM images of pristine hBN

and BNNSs, respectively. Pristine hBN exhibits as two-dimensional self-standing platelets with lateral dimen-sion of about 0.5~5 μm and initial thicknesses morethan 100 nm. In contrast, owing to effective exfoliation,BNNSs lie flat on the substrate and the top layers aretransparent to electron beams to see the bottom layers(Fig. 3b). The atomic force microscopy (AFM) image(Fig. 3c) shows most of the exfoliated BNNSs are lessthan 5 nm in thickness. The morphology of theseBNNSs was further characterized by TEM (Fig. 3d–f ).As observed in Fig. 3d, several very thin BNNSs coverthe supporting film, whose morphology is similar to theSEM image. Figure 3e demonstrates an exfoliated five-layer-atom BNNS with thickness of about 1.8 nm. Itsinterplanar spacing is measured as 0.35 nm, correspond-ing to the (002) plane. The selected area electron diffrac-tion (inset in Fig. 3e) reveals the good sixfold symmetryof BNNSs, indicating that BNNSs are structurally inte-gral and not damaged during ultrasonic exfoliation.High-resolution TEM image (Fig. 3f, left) together withits inverse fast Fourier transform (IFFT) (Fig. 3f, right)

confirms the hexagonal atom configuration of BNNSs,and the inset in IFFT image indicates that the center dis-tance between adjacent hexagonal rings is 0.25 nm [14].Figure 4a gives the XRD pattern of pristine hBN and

BNNSs. The hexagonal phase of pristine hBN is charac-terized by the peaks at 2θ = 26.8°, 41.7°, 43.9°, 50.2°, and55.2°, which correspond to the (002) (d002 = 0.33 nm),(100), (101), (102), and (004) planes, respectively. Incontrast, these peaks of BNNSs show a reduced inten-sity and sharpness, which correlated with their weak-ened c-direction stacking [29]. A slight shift of (002)peak from 2θ = 26.8° (hBN) to 26.2° (BNNSs) indicatesan increased layer interspacing (d002 = 0.35 nm).Furthermore, the inset reveals that the intensity ratio of(004) peak to (100) peak, I004/I100, of BNNSs is 0.316,far less than that of hBN (0.802), which can be inter-preted by the preferred orientation of exfoliated (004)or (002) plane [26]. Figure 4b shows the Fourier trans-formation infrared spectroscopy (FTIR) spectra ofpristine hBN and BNNSs. The hBN has two character-istic peaks at 1389 and 803 cm−1, presenting the in-plane B–N stretching and out-of-plane B–N bendingvibration, respectively. They blue shift to 1395 and810 cm−1 when the hBN was exfoliated into BNNSs.This shift can be attributed to the thinning of hBN afterexfoliation, which enhances the stretching vibrationand specially bending vibration of B–N bonds. Inaddition, the weaker band at ~3400 cm−1 is correlatedwith O–H or N–H stretching vibrations or absorbedwater molecules, widely observed in BN materials. TheRaman spectra were presented in Fig. 4c. The strongpeak at 1366.8 cm−1 presents the high-frequency inter-layer Raman active E2g mode of pristine hBN. Upon ex-foliation, it red shifts to 1363.8 cm−1 with an increasedfull width at half maximum, implying the reduced inter-layer interaction of exfoliated products [32, 33], whichagrees with the XRD and FTIR results.

Fig. 2 a Dependence of the yield and suspension concentration of BNNSs exfoliated in MEA aqueous solution on mass percents of water andthe corresponding specific surface tension of the solution. b Normalized absorbance of MEA-30 wt% H2O- and NMP-30 wt% H2O-exfoliatedsuspensions vs. storage time

Zhang et al. Nanoscale Research Letters (2017) 12:596 Page 3 of 7

The chemical and bond composition of BNNSs was fur-ther characterized by XPS (Fig. 5). XPS survey (Fig. 5a)shows the coexistence of B and N as main elements andO and C as impurities in the samples. The B/N ratio isabout 1.07, close to the reported values [16, 34]. In theB1s spectrum (Fig. 5b), the peak can be fitted using twocomponents: B–N bond (190.4 eV) and B–O bond(191.2 eV). The latter may be formed due to the hydrolysis[35] of B atoms in defected BN layers or the adsorbedH2O into BN layers during exfoliation. In the N1sspectrum (Fig. 5c), the small peak in 399 eV is assigned toN–H bond, which is possibly introduced together inhydrolysis. The hBN exhibits similar XPS spectra(Additional file 1: Figure S1) except that there is a slightly

reduced B–O bond contribution in B1s peak and absenceof N–H bond contribution in N1s peak.As an application, we prepared ER-BNNS composite

by dispersing the BNNSs in ER polymer. Figure 6 con-ducts the DMA (a, b) and mechanical tests (c) for theobtained ER-1% BNNS composite and pure ER. Figure 6ashows that the storage modulus E′ of the composite ishigher than pure ER in the glassy state, indicatingenhanced rigidity of the composite. This can be ascribedto the introduction of rigid PVP-functionalized BNNSs,as well as the strong interaction between the BNNSs andER matrix. Figure 6b gives the dependence of loss modu-lus tanσ of pure ER and ER-1% BNNSs on temperature,where the temperature corresponding to loss peak

Fig. 4 a XRD, b FTIR, and c Raman analysis of pristine hBN and BNNSs

Fig. 3 SEM images of pristine hBN (a) and BNNSs (b), AFM image (c), TEM images (d–e) of BNNSs, and high-resolution TEM image (f) of a BNNS(left) and its IFFT image (right)

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indicates glass transition temperature Tg [36]. It showspure ER has a Tg peak at 130 °C with an intensity of0.28. After addition of 1 wt% PVP-functionalized BNNSs(BNNSs-PVP) in ER, the Tg peak shifts to 165 °C withincreased intensity of 0.58. This suggests that two-dimensional BNNSs can, on the one hand, effectivelyrestrict the segmental motion and relaxation of ERpolymers by space limit and interface bonding so thatthe Tg of composite increases and, on the other hand,create numerous heterointerfaces throughout the matrix,leading to the increases of stress loss. To evaluate thereinforcement effect of the BNNSs, Fig. 6c compares tensilestrength σs and Young’s modulus Y for pure ER and ER-1%BNNSs. It shows σs = 64.25 MPa and Y = 1.3 GPa for pureER, while σs = 73.5 MPa and Y = 2.01 GPa for ER-1%BNNSs. That is, an addition of only 1 wt% BNNSs-PVPincreases σs and Y of the ER by 14.4 and 53.8%, respectively.The property comparison suggests that our ER-1% BNNScomposite is superior to most BN-filled polymers reportedin literatures (Additional file 1: Table S3).

ConclusionsIn summary, we reported on MEA aqueous solution as anew type of mixed solvents for high-efficient and cost-effective liquid exfoliation of BNNSs. The controlexperiments show MEA can exfoliate hBN superior thancurrently known solvents, and this ability can be further

improved by the addition of water of appropriateamount in MEA. In the optimum, an exfoliation yieldmore than 40% was achieved in MEA-30 wt% H2O solu-tion. Also, we found that this solution, when resulting inthe most efficient exfoliation of BNNSs, has a much highSST which deviated greatly from the predictions bySPTs, suggesting that additional interactions might needto be considered in SPTs to better interpret the liquidexfoliation. The exfoliated BNNSs demonstrate an abilityof significantly improving the thermal and mechanicalproperties of polymers. The mixed solvent here reportedenables the scalable exfoliation and applications ofBNNSs and exhibits great potentials in other exfoliationtechniques, such as shear exfoliation and ball millingexfoliation, and other two-dimensional materials.

MethodsMaterialshBN powder (1~5 μm, 99.5%), monoethanolamine (MEA),N-methyl-2-pyrrolidone (NMP), isopropanol (IPA),dimethylformanmide (DMF), tert-butanol (tBA), polyvinyl-pyrrolidone (PVP, molecular weight ~8000), methylhexahy-drophthalicanhydride (MeHHPA), and 2,4,6-tris(dimethyl-aminomethyl) phenol (DMP-30), purchased from Aladdinindustrial corporation of Shanghai, were of reagent grade.Bisphenol-A epoxy resin (epoxide number 0.48~0.54) wasprovide by Baling Company, SINOPEC.

Fig. 5 XPS spectra of a BNNS survey, b B1s, and c N1s

Fig. 6 a Storage modulus E′, b loss modulus tan σ, and c tensile strength σs and Young’s modulus Y of pure ER and ER-1% BNNSs

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Preparation of BNNSsTypically, 200 mg of pristine hBN powders was mixed in50 mL of MEA or MEA aqueous solution with givenwater content in a 200-mL beaker, before sonicated for4 h at about 50 °C in a 6-L bath sonicator (KQ3200DA,Kunshan Shumei) operating at 40 kHz and suppliedpower dissipation of 150 W. The resultant suspensionwas centrifugated at 3500 rpm for 20 min. The super-natant was decanted to afford a concentrated solution ofexfoliated BNNSs. It was washed with ethanol repeatedlyand vacuum dried at 100 °C overnight, giving the BNNSpowder. The yield is defined as the mass ratio ofexfoliated BNNSs to pristine hBN. For comparison,several popular solvents such as NMP, DMF, IPA, andtBA were chosen to exfoliate hBN powders following thesame process.

Preparation of ER-BNNS CompositeFirst, 30 mg BNNS powder and 100 mg PVP weredispersed in 10 mL of DMF. Then the dispersion wasstirred at 100 °C for 6 h, allowing PVP to adhere to thesurfaces of BNNSs. The resultant suspension was sepa-rated, washed, and dried following the above procedure,giving PVP-functionalized BNNSs (BNNSs-PVP). Third,bisphenol-A epoxy resin, MeHHPA, and BNNSs-PVP(41:57.5:1 by mass ratio) were mixed for 40 min beforebeing vacuum degassed at 60 °C for 20 min; the mixturewas added with 0.5% DMP-30 as the promoter and thensonicated for 10 min. Finally, the resulting paste wascasted in a mold and cured using a heating proced-ure: 80 °C/10 h + 100 °C/3 h + 150 °C/3 h, formingthe ER-1% BNNS composite sheets as testing samples.For comparison, pure ER samples were obtained usingthe above process in the absence of BNNSs-PVP. Thesamples are tape-like (DMA test) with a size 10 mm ×25 mm × 1 mm or dumbbell-like (mechanical test)with thickness of 1 mm.

CharacterizationOptical absorption spectra were taken from a spectro-photometer (UV-vis; Persee T1910). The chemicalcomponents were analyzed using Fourier transform-ation infrared spectroscopy (FTIR; Bruker IFS66V),Raman spectrometry (RS; HORIBA JY, LabRAMX-ploRA ONE), and X-ray photoelectron spectroscopy(XPS; Kratos Axis Supra, Al-Kα radiation). Thephases were identified by X-ray diffraction (XRD;PANalytical, X’Pert PRO, Cu-Kα radiation, 1.54 Å).The morphology and size of nanosheets wereobserved using field-emission scanning electronmicroscopy (SEM; Hitachi, S4800), transmission elec-tron microscopy (TEM; JEOL, JEM-2010), and atomicforce microscopy (AFM; Bruke, Dimension Icon). ForER/BNNS samples, dynamic mechanical analysis was

carried out with a dynamic mechanical analyzer(DMA; DMA8000, Perkin Elmer) based on a singlecantilever mode at a frequency of 1 Hz. Tensilestrength and Young’s modulus were measured usingan electronic universal testing machine (CMT-200,Jinan Liangong) with a load range of 0~200 kN.

Additional file

Additional file 1: Table S1. Liquid exfoliation comparison of BNNSsand other two-dimensional materials. Table S2. Comparison of BNNSsexfoliated by various methods. Table S3. Property comparison of variousBN/polymer composites. (DOC 2240 kb)

AcknowledgementsWe would also like to thank Zemin Yuan, doctoral student in theDepartment of Functional Materials Research, Central Iron and Steel ResearchInstitute of China, for his assistance with the TEM and AFM analyses.

FundingThe authors are grateful for the financial support of the National NaturalScience Foundation of China under grant 51462028.

Availability of Data and MaterialsBoron nitride nanosheets (BNNSs), monoethanolamine (MEA), epoxy resin(ER), N-methyl-2-pyrrolidone (NMP), isopropanol (IPA), dimethylformanmide(DMF), tert-butanol (tBA), polyvinylpyrrolidone (PVP), methylhexahydrophtha-licanhydride (MeHHPA), 2, 4, 6-tris(dimethyl-aminomethyl) phenol (DMP),Fourier transformation infrared spectroscopy (FTIR), Raman spectrometer (RS),X-ray photoelectron spectra (XPS), X-ray diffraction (XRD), scanning electronmicroscopy (SEM), transmission electron microscopy (TEM), atomic forcemicroscopy (AFM).

Authors’ ContributionsBZ and QW conceived the project and designed the experiments. BZ, HS,and HY carried out the experiments and characterizations. BZ, RL, XG, and RXtook part in the analysis of the results, BZ and HY wrote the paper, and all ofthe authors read and revised the paper. All authors read and approved thefinal manuscript.

Competing InterestsThe authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.

Received: 20 September 2017 Accepted: 6 November 2017

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